This invention relates to a method and apparatus for the treatment of aqueous liquid having an oxygen demand.
Conventional waste water treatment is carried out in a vessel by aerobic bacterial solids that degrade organic contaminants in the water in the presence of oxygen. The water in the vessel is agitated to keep the bacterial solids in suspension. Aeration of the water maintains the necessary aerobic conditions.
On a conventional waste water treatment plant there is generally a continuous flow of water to be treated into the vessel and a continuous flow of treated water containing bacterial solids out of the vessel. The outward flow of treated water is directed from the treatment vessel to a settling tank in which the solids settle out under gravity, leaving a clear supernatant liquid that can be continuously discharged to the environment or for further treatment.
It has been proposed to do away with settlement under gravity and employ cross flow membrane filtration to separate the bacterial solids from the water. One potential advantage of membrane filtration is that it can successfully handle higher concentrations of bacterial solids than gravity separation. As a result stronger aqueous waste materials can be treated. The ‘strength’ of an aqueous waste material is reflected by its Biological Oxygen Demand (BOD) or its Chemical Oxygen Demand (COD), or both.
So far to date, it has proved difficult to obtain the maximum benefit from tubular cross flow membrane filters in a waste water treatment process. Difficulties arise in keeping the membrane's inner bore surfaces clean and therefore in exploiting their potential ability to handle high strength solid concentrations. These former difficulties can be mitigated by scouring the inner bore surfaces with bubbles of air as in WO01/00307A. However the scouring method described is in isolation without there being any attempt to integrate such scouring into a total aqueous liquid treatment process, the main challenge being to maintain suitable treatment conditions in the treatment vessel.